"We realized a few years ago that the value of switching to Slack
was so obvious and the advantages so overwhelming that every
business would be using Slack, or 'something just like it,'
within the decade. It's validating to see you've come around to
the same way of thinking. And even though — being honest
here — it's a little scary, we know it will bring a better future
forward faster.

"However, all this is harder than it looks. So, as you set out to
build 'something just like it,' we want to give you some friendly
advice."

The letter goes on to say that "Slack is here to stay" and
welcomes Microsoft "to the revolution":

"We're glad you're going to be helping us define this new product
category. We admire many of your achievements and know you'll be
a worthy competitor. We're sure you're going to come up with a
couple of new ideas on your own too. And we'll be right there,
ready."

According to recent reports, Microsoft was considering an
$8 billion bid for Slack, but Microsoft
founder Bill Gates and current CEO Satya Nadella opposed the deal
in favor of developing an in-house solution, before an offer
could be made.

Now Microsoft is launching its work-chat app called Teams, a chat
app that — much like Slack — offers "channels," which are
different chat rooms that work teams can set up for themselves. A
video introducing Teams is now up online:

The Slack ad appears to be a riff on an Apple ad from 1981 welcoming IBM to the
personal-computer market and reminding the company that Apple
was there first. That ad ran as a full page in The Wall
Street Journal. And even that ad apparently channeled one from an
early IBM competitor, Data General, after IBM entered the
minicomputer industry.